Our objective is to develop in the dog procedures for securing permanent survival of allografts of large organs that are adaptable to clinical use. Our area of study is the tolerance to organ grafts that can be induced following total body irradiation and the implantation of marrow. A preliminary series of 20 experiments, 8 kidney transplants, and 12 lung transplants performed with our surgical colleagues F. Rapaport, New York University-kidney, and D. Blumenstock, The Mary Imogene Bassett Hospital, Columbia University - lung, has demonstrated that we can select a donor of marrow, and a donor of organ -- other than the donor of marrow -- irradiate the proposed recipient, transplant the marrow and the lung or kidney and secure a healthy animal with a functioning and unrejected graft. The donor of marrow can even be the irradiated dog himself, whose marrow is aspirated and preserved in ice box culture for the 12 to 24 hours required for the dog's irradiation and operation. The preliminary series has been done with subjects and donors matched for major DL-antigens. We wish to explore its reproducibility and the latitudes of histocompatibility permitted, together with the time sequences and durations allowable in the operation, irradiation, and marrow implantation procedures. We wish also to study the role of preliminary thymectomy of the subject in order to have the fullest possible understanding of the latitudes permissible in making clinical application of these procedures for the permanent and satisfactory grafting of organs.